100 PROCESS OF NUTRITION. CHAP. III. 



ing its inadequacy, or of making it appear that 

 there is any absurdity in the doctrine it was in- 

 tended to support; but rather that the doctrine, 

 though founded in truth, could not have been satis- 

 factorily proved by any experiments that were prac- 

 ticable at the time. 



Their in- It is to the modern improvements in pneumatic 

 proved by chemistry, and to them alone, that we are indebted 

 chemTcT for our knowledge of the real functions of the 

 experi- leaves of plants ; from which it is proved indispu- 

 tably, that the leaves not only contain air, but do 

 actually inhale it. It was the opinion of Priestley 

 that they inhale it chiefly by the upper surface. 

 Has this been confirmed ? And it has been 

 shown by Saussure that their inhaling power de- 

 pends entirely upon the organization. A bough 

 of the Cactus Opuntia, when placed, as it was de- 

 tached from the plant, in an atmosphere of common 

 air, inhaled in the course of a night four cubic 

 inches of oxygene ; but when it was placed in a 

 similar atmosphere after being cut to pieces and 

 pounded in a mortar so as to destroy the organiza- 

 tion of its parts, no inhalation took place. The 

 inhalation of air, therefore, is no doubt effected by 

 the pores of the epidermis of the leaf. 



It has been a question, however, among phytolo- 

 gists, whether it is not also effected by the epidermis 

 of the other parts of the plant. We can scarcely 

 suppose it to be effected by the dry and indurated 

 epidermis of the bark of aged trunks, of which 



